Digital cameras and video cameras with a solid state imaging device of either CCD type or CMOS type are in widespread use. Often, the solid state imaging device, together with a memory device, is incorporated in electrical equipments such as personal computers, mobile phones, and personal digital assistances to provide them with an image capturing function. An increasing demand for the solid state imaging devices is downsizing so that it does not affect the size of the electrical equipments greatly.
Solid state imaging device of chip scale package type (hereinafter “CSP”) are known in the art. The CSP type solid state imaging device is composed of, for example, an imaging chip provided with both an image sensor (such as CCD) and input/output pads on the upper surface, a cover glass (i.e. cover) attached on the upper surface of the imaging chip through a spacer for sealing the imaging sensor, and external electrodes for connection to external devices. The CSP type solid state imaging device is packaged in approximately the size of the imaging chip, which is very small (see, for example, the Japanese patent laid-open publication No. 2001-257334).
Generally, the CSP type solid state imaging device is made in the following procedure:
1. forming a plurality of the image sensors (each with a photoelectric conversion section and a charge transfer section) and the corresponding input/output pads on the upper surface of a silicon wafer.
2. forming the spacers atop of the silicon wafer to enclose the image sensors individually.
3. attaching a glass substrate, i.e. a base material of the cover glass, to the silicon wafer through the spacer to seal each of the image sensors.
4. forming on the silicon wafer the external electrodes that correspond to the image sensors.
5. dicing the silicon wafer and the glass substrate into individual image sensors.
Since the CSP type solid state imaging device is as small as the imaging chip, the external electrodes are usually formed on the lower surface of the imaging chip. Therefore, with the conventional technique, the external electrodes on the lower surface are connected to the input/output pads on the upper surface of the image sensor in the formation process of the external electrodes. Concretely, they are connected either by through wirings that pass through the imaging chip or side wirings formed on the side faces of the imaging chip.
The external electrode with the through wiring may be formed in the following procedure (see, for example, “Press release, a next generation packaging method with a through electrode on a semiconductor chip is established at practical level” from Association of Super-Advanced Electronics Technologies, searched on Feb. 15, 2005, via the Internet, <URL:http://www.aset.or.jp/press_release/si—20040218/si—20040218.html>):
1. forming the through holes on the silicon wafer to extend from the lower surface to the input/output pads.
2. forming insulating thin layer on the inner walls of the through holes.
3. forming the through wirings of copper plate in the through holes.
4. forming, on the lower surface of the silicon wafer, secondary wirings to be connected with the through wirings.
5. forming solder balls on the secondary wirings.
On the other hand, the external electrode with the side wiring may be formed in the following procedure (see, for example, the U.S. Pat. No. 6,777,767 B2):
1. attaching a reinforcing board to the lower surface of the silicon wafer.
2. forming secondary wirings on the lower surface of the reinforcing board.
3. forming cutouts, which pass through between the lower surface of the reinforcing board and the input/output pads, at the portions that would become the side faces of the solid state imaging device.
4. forming conductive films inside the cutouts to connect the input/output pads and the secondary wirings.
5. forming solder balls on the secondary wirings.
When the silicon wafer is diced, the conductive films appear on the side faces of the solid state imaging device and become the side wirings.
Forming of the above through wirings requires a large number of processes, such as etching, film formation, and plating. Moreover, certain dedicated machines (for example, the etcher, the CVD machine, the plating machine) are required, which will raise the cost. Also required is much time for listing and evaluating the conditions, such as the shape of the through hole, the thickness of the insulating film and a conductive paste and so force, in order to ensure a certain level of the electric property and the reliability.
For the same reason, forming of the side wirings also raises the cost. Moreover, the side wirings cause to enlarge the solid state imaging device because the input/output pads have to be spaced at no less than 350 μm interval for the wires passing through the side faces to the lower surface. When it is enlarged, fewer solid state imaging devices can be produced from a single silicon wafer and the cost will be raised.
The Japanese patent laid-open publication No. 2001-257334 discloses a packaging method of the CSP type solid state imaging device. This method begins by fixing a sensor package (composed of an imaging chip and its cover glass) without external electrodes onto a circuit board made from a glass epoxy substrate. Then, input/output pads of the sensor package and conductive pads of the circuit board are joined by bonding wires. Finally, the periphery of the sensor package is sealed with sealing resin. This packaging method enables to use the conventional post-process technique and facilities for semiconductor devices.
Although it becomes somewhat large, this kind of resin sealed package enables to produce the reliable solid state imaging devices at low cost, and solve the above mentioned drawbacks of the through wirings and the side wirings as well. In addition, the sealing resin seals a cavity enclosed by the imaging chip, the spacer, and cover glass, preventing electrical problems in the image sensor.
However, the Japanese patent laid-open publication No. 2001-257334 is silent about how to protect the upper face of the cover glass in the sealing process. If the upper face of the cover glass is overlaid, stained, or damaged by the resin, the yield is decreased and the cost is raised. Therefore, a development of an effective resin sealing method able to protect the upper surface of the cover glass is highly demanded.
In the meanwhile, one of the interests of the art is a so-called “system-in-package” (hereinafter “SIP”), which packs a plurality of IC chips, normally mounted separate on the circuit board, into a single package as a system. For the SIP of a CPU and memories, there is no need to expose the IC chips. In contrast, when the above sensor package is packed in the SIP, the cover glass has to be exposed and, therefore, care must be taken for arrangement of the sensor package.
While the resin sealed sensor packages have been used in various semiconductor devices, they are known for a problem of moisture absorption by the sealing resin. This problem occurs when the semiconductor device in the resin seal package is soldered by a solder reflow machine. The moisture in the sealing resin is heated and causes a steam explosion, which makes some cracks in the sealing resin or at the boundary of the sealing resin and the sensor package. Therefore, the sealing resin peels off from the sensor package or the circuit board.
If this problem occurs in the solid state imaging device of the Japanese patent laid-open publication No. 2001-257334, the cracks or the peeling off of the sealing resin lead to remove the bonding wires connected to the input/output pads and the conductive pads. At the end, the solid state imaging device becomes inoperative. In addition, the moisture in the sealing resin dissolves ionic impurities out of the sealing resin, and an electrochemical decomposition reaction is thereby stimulated to possibly corrode the aluminum-made input/output pads. Furthermore, when the sealing resin peels off from the sensor package or the circuit board, the spacer and the cover glass may also come off, along with the sealing resin, from the imaging chip. This will cause other problems, such as fog on the cover glass and electrical malfunctions of the image sensor.
Despite the fact that the moisture absorption of the sealing resin causes various problems to the solid state imaging devices as described above, the Japanese patent laid-open publication No. 2001-257334 is silent about a solution to the moisture related problems. Therefore, a proper countermeasure to the moisture of the solid state imaging devices is highly demanded.
In view of the above, an object of the present invention is to provide a manufacturing method of the solid state imaging device that enables to seal the sensor package with resin while effectively protecting the cover glass for the image sensor. This manufacturing method of the solid state imaging device can also prevent negative effects of moisture. The resulting solid state imaging devices are also within the scope of the present invention.